The invention relates to birefringent glasses and their use in making waveplates.
Waveplates, also called linear phase retarders or retardation plates, introduce a phase shift between polarized components of light transmitted through the plate. The birefringent property of the waveplate causes the light to split into an ordinary ray and an extraordinary ray. The two rays travel at different velocities in the plate. The path difference, kλ, expressed in wavelengths, between the two rays is given by:                               k          ⁢                                          ⁢          λ                =                  ±                      l            ⁡                          (                                                n                  e                                -                                  n                  o                                            )                                                          (        1        )            where ne is the refractive index of the extraordinary ray, no is the refractive index of the ordinary ray, l is the physical thickness of the waveplate, k is the wavelength of the light ray, and k can be considered as the retardation expressed in fractions of a wavelength. The difference in velocities of the rays result in a phase difference, also called plate retardation, when the two rays recombine. The phase difference, δ, between two rays traveling through a birefringent material is 2π/λ times the path difference. That is,                     δ        =                              ±                                          2                ⁢                π                            λ                                ⁢                      l            ⁡                          (                                                n                  e                                -                                  n                  o                                            )                                                          (        2        )            
Waveplates are characterized based on the phase difference introduced between the ordinary and extraordinary rays. For a half waveplate, δ=(2m+1)π, i.e., an odd multiple of π. For a quarter waveplate, δ=(2m+1)π/2, i.e., an odd multiple of π/2. For a full waveplate, δ=2mπ. For the full, half, and quarter waveplates, the order of the waveplate is given by the integer m. When m=0, the term zero order waveplate is used. When m>0, the term multiple order waveplate is used. For waveplate applications requiring high stability, a low order, and ideally zero order, waveplate is preferred. In this respect birefringent glasses, such as disclosed in U.S. Pat. Nos. 5,375,012 and 5,627,676, have an advantage over crystalline materials such as quartz, calcite, and mica. With birefringent glasses, zero order waveplates can be made in an integral body with a practical thickness for finishing and handling, e.g., 0.5 to 1.5 mm thickness in the visible wavelength range. Crystalline materials such as mentioned above require zero order waveplates to be impractically thin, e.g., on the order of 25 μm, and are typically better suited for making higher order waveplates.
U.S. Pat. Nos. 5,375,012 and 5,627,676 teach that a birefringent glass can be produced by applying stress to a phase-separated glass at an elevated temperature. A phase-separated glass is a glass which, upon heat treatment, separates into at least two phases: a separated phase in the form of particles, either amorphous or crystalline, dispersed in a matrix phase. The applied stress elongates the particles and generates a form birefringence in the glass. U.S. Pat. No. 5,375,012 discloses that the phase-separated glass could be selected from a glass containing silver halide particles, PbO—B2O3 glasses (and borosilicate glasses with high B2O3 contents) that tend to exhibit a secondary borate phase, and bivalent metal (lead, calcium, barium and strontium) oxide, silicate and borosilicate glasses. U.S. Pat. No. 5,627,676 discloses a phase-separated glass having crystalline particles selected from the group consisting of copper chloride, copper bromide, and mixtures thereof dispersed in a R2O—Al2O3—B2O3—SiO2 glass matrix. U.S. Pat. No. 5,627,676 reports that the degree of form birefringence obtainable in a glass containing copper bromide and/or chloride particles is substantially greater than that obtained in a silver halide glass.
The ability to obtain form birefringence in a stretched phase-separated glass is not unusual especially when the phase separation is liquid—liquid in nature. The down side is that invariably, the index ratio of the separated phase to the matrix phase is small, resulting in a correspondingly small birefringence. In the phase-separated glass containing silver halide particles, the index ratio of the separated phase to the matrix phase is not the problem, but the amount of silver halide phase that can be produced is limited, which ultimately limits the magnitude of form birefringence that can be achieved. It is possible to increase the amount of silver halide phase by using a glass composition with a higher silver content; however, this approach seems to have reached its limit with the result of a half wave at 1500 nm in 1.6 mm thickness. Therefore, in one extreme situation simple liquid—liquid phase separation can attain high volume fractions of the separated phases but with small index contrast. In the other extreme situation, liquid—liquid phase separation has high index contrast but limited amount of the separated phase.
From the foregoing, what is desired is a glass composition that can produce liquid—liquid phase separation with high volume fraction of the separated phase and high index contrast between the separated phase and the matrix phase.